The normal growth cycle of hair lasts anywhere from 2 to 5 years and consists of three phases. The anagen phase is the first and longest lasting phase in which hair is actively growing. During the catagen phase, which lasts only a few weeks, the hair root moves toward the scalp. Finally, the telogen phase is a resting period that ends when the hair falls out and the anagen phase starts again.
At any given time, each hair is in a different stage in the growth cycle, resulting in the maintenance of a full head of hair. However, during male pattern baldness, there is a shortened anagen, or growing phase. Hair grows to a shorter length and falls out more frequently, eventually causing bald spots on the head, most often beginning at the temple and crown of the head.
Hair loss can be hereditary, but it can also be related to stress, excess UV radiation, medication side effects, elevated body temperature, or hormonal changes.